Ferguson Fire: Tree mortality epidemic adding to fire...

1of3Photo 1: U.S. Forest Service Fire Behavior Analyst Robert Scott near the leading edge of the Ferguson Fire close to Lushmeadows on Thursday, July 26, 2018.Photo 2: Jason Engle, left, a U.S. Forest Service Ranger , and Forest Service Fire Behavior Analyst Robert Scott evaluate the Ferguson Fire near Lushmeadows, Mariposa County.Photo: Kurtis Alexander / The Chronicle

3of3The inside of the bark from a ponderosa pine that is infested by the Western pine beetle.Photo: Michael Macor / The Chronicle 2016

LUSH MEADOWS, Mariposa County — The thousands of firefighters doing battle just outside of Yosemite National Park on Thursday braved not only steep hills, 90-degree temperatures and tinder-dry brush but towering stands of easily ignitable dead trees.

The Ferguson Fire, which this week shut down much of Yosemite, is burning near the heart of California’s tree mortality epidemic. Years of drought and bark beetle infestation have left millions of lifeless trunks languishing across the state, with the greatest concentration in the southern Sierra.

As the fire threatened several rural communities near Mariposa, dead trees provided ready kindling for the blaze, and sometimes produced explosive blasts of unexpected heat. Forestry experts worry that the tree die-off is intensifying this summer’s California wildfires during what’s already become a difficult firefighting season.

“We always have someone keeping a lookout for those trees,” said Jason Engle, a U.S. Forest Service ranger from Arkansas assigned to the fire in the Sierra National Forest. “The trees catch fire and come down real easily.”

Just above the small enclave of Lush Meadows, which is about 6 miles west of Yosemite, flames had found their way over a wooded mountainside known as Footman Ridge, where several tall dead pines poked out of the forest canopy. Crews had dug a break on the ridgetop to stop the fire, but that proved unsuccessful. Now, 16 helicopters and scores of firefighters were trying to stifle what had become the most aggressive front of the 44,223-acre blaze. A handful of homes stood below.

“All those big smokes you see up there are (the result of) fallen logs,” said Robert Scott, a Forest Service fire behavior analyst who has been monitoring and forecasting the fire’s progress. “As the dead trees catch fire, they’re falling and sliding down the hill and starting new fires. If there weren’t the dead trees, it wouldn’t be doing that.”

The good news, Scott said, was that most of the other sides of the fire weren’t nearly as active. Higher humidity and less hot sunshine, due to the smoke cover, were slowing the fire’s expansion.

“This is going to give the operations people time to get in and put in more breaks,” he said.

The Ferguson Fire, which began July 13, was estimated to be 27 percent contained Thursday as more than 3,800 firefighters worked to squash it. No homes have burned, but hundreds of residents, mostly east of Mariposa, have evacuated. More than 5,000 structures are threatened in the areas where evacuations were advised.

Gov. Jerry Brown late Thursday declared a state of emergency in Mariposa County, making it easier for fire managers to get more money and manpower.

One firefighter, heavy equipment operator Braden Varney, died while on duty and seven others have been injured.

Due to heavy smoke and firefighting activity, the National Park Service on Wednesday ordered a seldom-seen shutdown of Yosemite Valley and the Wawona area of the park. Only Highway 120 through Tuolumne Meadows and the Hetch Hetchy area remained opened. The closures are expected to last until Sunday.

Mariposa County is one of the parts of the state hit hardest hit by California’s tree die-off. The U.S. Forest Service estimates that at least 129 million trees have fallen prey to the drought and bark beetle since 2010. Warmer, drier conditions have weakened forests and allowed the beetle to flourish.

Related Stories

Last year’s rainy winter is believed to have slowed the growth of the beetle population, but it remains to be seen by how much. In the meantime, many of the affected trees are dying.

“We’re starting to see more trees fall down now,” said Amarina Wuenschel, ecologist for the Sierra National Forest.

And when that happens, she said, there’s more on-the-ground fuel for fire to devour.

The state of the dying trees is also a factor in fire behavior. For example, when those trees have lost their needles, Wuenschel said, they won’t burn much at their crowns. A live tree, however, can be more dangerous, although a dead tree still bearing needles may light faster.

“Greater continuity of fuels from tree mortality is expected to increase (the) rate of fire spread and potentially fire intensity,” she said.

While the science is still undecided about how much worse the dead trees can make a fire, crews are being cautious about the potential dangers.

Above Lush Meadows, firefighters were clearing dead trees before establishing breaks and before extinguishing the fire in places where the blaze had burned to the break.

“You have to have dozers and firefighters with saws in there first,” Scott said. “It just takes a little more energy.”

Kurtis Alexander is a general assignment reporter for The San Francisco Chronicle, frequently writing about water, wildfire, climate and the American West. His recent work has focused on the impacts of drought, the widening rural-urban divide and state and federal environmental policy.

Before joining the Chronicle, Alexander worked as a freelance writer and as a staff reporter for several media organizations, including The Fresno Bee and Bay Area News Group, writing about government, politics and the environment.